1. Field of the Invention
The present invention relates to methods of and devices for cold drawing seamless metal tubes, and more particularly to methods of producing the seamless metal tubes by cold drawing and to draw benches for effecting the methods, the seamless metal tubes each having an upset portion on both ends.
These seamless metal tubes are preferably used for drilling tubes, casings, inner rods and outer rods for geological, mineralogical, metallurgical or geothermal research and/or drilling for civil engineering or water wells.
2. Prior Art
While reference to FIGS. 1(a)-4(b) showing the seamless metal tubes which have been produced by the conventional step and the conventional draw bench, a hot forged thin wall midbody 7c of a metal tube is welded at each end with a thick wall end tube 7b in order to obtain a larger diameter seamless metal tube or pipe 7 having an upset portion 7b on both ends.
Otherwise, the seamless metal tube is produced by casting or lathing, but metallic filaments in such a seamless metal tube are axially cut at welded or lathed portions or at blowholes, which are likely to cause cracks in the seamless metal tube after heat treatment at high temperature.
To this end, a draw bench for drawing the seamless metal tube has been proposed. For example, a small diameter and thin wall seamless metal tube having a diameter of 25-35 mm and a wall thickness of 2.1-3.2 mm such as for a bicycle framework is cold drawn through the draw bench shown in FIGS. 2(a)-4(b).
The conventional draw bench, which is shown in FIGS. 2(a) and 2(b), for drawing such seamless metal tubes comprises a bed 12 to be fixed on the floor, a die holder 26 which is mounted at a middle portion on the bed 12 and a closed periphery reducing die 2 which is concentrically mounted in the die holder 26. A pair of tongs 6 and a bearing are arranged on the entry side of the bed 12 for horizontally supporting a plug fixing rod 88.
Secured to a front portion of the rod 88 is a plug 4 which is inserted into the reducing die 2 when drawing.
A draw unit 110 is arranged on the exit side of the bed 12 and comprises a carriage 112 mounted on rollers travelling on the bed 12, the carriage 112 carrying a hook 118 and a chuck 114 for gripping the front tip 7a of the mother tube or workpiece 7. The carriage 112 is driven by an endless chain 120 encompassing a driven sprocket wheel (not shown) mounted in the bed 12. The driving sprocket wheel is coupled by any known means with an electric motor (not shown). The hook 118 is engaged with the endless chain 120 when drawing. The front tip 7a of the mother tube 7 engaged by jaws 116 of the chuck 114 is strongly pulled by the carriage 112 to draw the tube 7 having an upset portion 7b on both ends.
FIGS. 3(a) and 3(b) show the conventional manner of drawing the seamless metal tubes which have the same outer diameter but unequal inner diameter upset portions on both ends. In FIG. 3(a), the plug 4 is held adjacent to a tapered entry in the axial bore of the reducing die 2 with a clearance to the inner periphery of the mother tube 7 to draw the portion of the tube 7 having the small inner diameter upset portion 7b on both ends, while the plug 4 in FIG. 3(b) is held right in the bore of the reducing die 2 to draw the portion of the tube 7 having a large inner diameter.
FIGS. 4(a) and 4(b) show another conventional drawing method. The reducing die 2 has an internal surface including an inlet portion 2b which tapers forwardly toward the bearing or throat portion 2a and an oppositely inclined outlet portion 2c. When the enlarged front portion 4a of the plug 4 is held right in the bearing portion 2a of the reducing die 2, the mother tube 7 is drawn through the bearing portion 2a and over the enlarged front portion 4a into a drawn tube having a small outer diameter.
In FIG. 4(b), the enlarged front portion 4a of the plug 4 is passed leftwards across the bearing portion 2a and located adjacent to the outlet portion 2c. Then the mother tube 7 is drawn through the bearing portion 2a and around the small diameter portion 4b of the plug 4 to reduce the outer diameter of the drawn tube.
The mother tube 7 is further advanced into the front region where the inner diameter of the mother tube 7 is widened by the enlarged front portion 4a of the plug 4 in order to draw the tube 7 having the same inner diameter as that of the enlarged front portion 2a of the plug 4 and also having an outer diameter upset portion 7b larger than that of the drawn tube 7 produced through the reducing die 2 shown in FIG. 4(a).
The seamless metal tube 7 drawn through the conventional reducing die 2 has the following disadvantages.
(1) The seamless mother tube 7 having a small inner diameter is drawn under reduction through the reducing die 2 shown in FIG. 3(a) without any internal radial pressure, thus resulting in corrugation in and around the drawn tube. PA0 (2) In FIG. 4(b), the mother tube 7 is drawn through the reducing die 2 with the plug 4 held in the bearing portion 2a and in the tube 7 to obtain the drawn tube 7 having one outer diameter but two different inner diameter upset portions 7b. The tube drawn through the bearing portion 2a and over and around the small diameter portion 4b of the plug 4 is widened by the enlarged front portion 4a of the plug 4 to obtain the drawn tube shown in FIGS. 7(a), 7(b) and 7(c), wherein PA0 (3) The configuration of the drawn tube is limited to only two kinds, i.e. PA0 (4) The drawing force of the plug 4 is so small that the plug 4 may be driven to transfer by a hydraulic cylinder, while it needs to provide a balancing unit or a plurality of hydraulic cylinders to balance the reaction upon the strong drawing force of the plug 4, thus making the device complicated and expensive.
d.sub.4 denotes a large diameter bearing portion of the plug 4, d.sub.2 denotes a bearing portion diameter of the reducing die 2, d.sub.1 denotes a large diameter of the drawn tube 7, and d.sub.3 a small diameter bearing portion of the plug 4 respectively. PA1 (a) one having one outer diameter but two unequal inner diameters; PA1 (b) another having one inner diameter but two unequal outer diameters.
Accordingly, d.sub.4 becomes an inner diameter of the drawn tube 7 and d.sub.2 becomes a small outer diameter of the drawn tube 7, but the large diameter d.sub.1 of the drawn tube 7 does not directly connect the bearing portion diameters of the reducing die 2 and the plug 4, but it gives the following functional formula, EQU d.sub.1 =f(d.sub.2, d.sub.3, d.sub.4).
But we cannot determine the values for d.sub.1, d.sub.2 and d.sub.4 independently. In order to fix the most preferable values for d.sub.1, d.sub.2 and d.sub.4, it is necessary to select the value sufficiently near the most suitable value among the various solutions of the functional formula of EQU d.sub.1 =f(d.sub.2, d.sub.3, d.sub.4).
We cannot, however, obtain the most suitable values for the diameters of d.sub.1, d.sub.2 and d.sub.4.